Zirconium oxide or zirconia is a widely used ceramic material. The high melting point (about 2600.degree. C.) and low thermal conductivity of zirconia make it attractive for refractory applications. When heated, zirconia displays a high diffusivity of oxygen which has led to its use in sensors which monitor the oxygen content of, for example, combustion gases. When small amounts of other metal oxides are alloyed with zirconia, stabilized zirconias can be prepared which possess extremely high fracture toughness. The preparation and properties of these stabilized zirconias are the subject of much research which has led to the use of zirconia in wear and structural applications. In addition, zirconia surfaces have been found to be catalytic for many chemical reactions and, therefore, its use as a catalyst or catalyst support is well known. It is particularly advantageous for many applications to have the fibrous material be in the form of continuous fibers.
The preparation of continuous oxide fibers by the dry spinning of concentrated metal oxide precursors is well known. By continuous is meant greater than 1.0 meter in length. The viscous, fiberizable dope comprising a metal oxide precursor and solvent, usually water or an alcohol, is extruded through an orifice under pressure and drawn, typically on a rotating take-up wheel. By dope is meant the viscous mass from which the fibers are formed by spinning, extrusion, drawing or blowing processes. The resulting fibers are referred to as "green" fibers. By "green" is meant unfired. The green fibers are subsequently heated to an elevated temperature to volatilize and remove fugitive species and to form the fired ceramic fiber. In commercial production of such fibers the mass throughput is an important economic consideration. The diameter of the fiber strongly affects the mass throughput at a give spinning rate and therefore the cost per given mass of fiber. For example, for a given fiber length a 10 micrometer diameter fiber has 4 times the mass of a 5 micrometer diameter fiber. Therefore, for equal spinning rates (m/min.) a fiber line producing a 10 micrometer diameter fiber will have 4 times the mass output of a fiber line preparing a 5 micrometer diameter fiber. A further advantage of larger diameter fibers is their smaller exterior surface area to volume ratio. Under conditions in which the surrounding environment reacts with the fiber, a larger diameter fiber will be more slowly attacked than a smaller diameter fiber. As the diameter of ceramic fibers increases, it becomes increasingly difficult to maintain a high specific strength. This is due to the statistical nature of brittle fracture. In addition, it is increasingly difficult to maintain a crack-free microstructure as fiber diameter increases. This is due to the increasing difficulty of both removing gases generated during pyrolysis and accommodating the shrinkage accompanying pyrolysis and sintering, and the consequent greater flaw frequency with increasing fiber diameter. In general, the specific strength of ceramic fibers decreases with increasing diameter to a greater extent than would be predicted by strictly statistical considerations which assume a constant microstructure.
A number of processes for the preparation of ZrO.sub.2 based fibers are known. One method is the "relic process" as described in U.S. Pat. Nos. 3,385,915 and 3,860,529. In the "relic process" the zirconium compound and compounds of any desired stabilizing oxides are impregnated into an organic polymeric fabric or textile. The impregnated fabric or textile is then heated in an oxidizing atmosphere. Fibers produced by this process, however, do not posses sufficient mechanical strength or flexibility for many applications.
Other processes for preparing ZrO.sub.2 based fibers have also used solutions of zirconium compounds such as salts or alkoxides as zirconium sources. Such processes are described in U.K. Patent Nos. 1,030,232 and 1,360,197; U.S. Pat. Nos. 3,180,741; 3,322,865; and 3,992,498. "Green" fibers are formed by spinning, drawing, blowing or extrusion. The green fibers are fired to volatilize and remove fugitives (i.e., water, organics, and anions such as chloride or nitrate) from the fiber and to form zirconium oxide. In these processes a solution of soluble zirconium salts, additives, and modifiers is concentrated, typically by warming under a reduced pressure in a rotary evaporator to produce a highly viscous fiberizable dope. During concentration of the solution to form the dope, the zirconium salts and possibly any additive metal salts may undergo hydrolysis or polymerization reactions to form hydrolyzed or polymerized species. These zirconium species may be of colloidal size and are referred to as sol particles. The nature of the sol particles which are formed "in-situ" in the dope are a function of a number of process variables including the anions present, the temperature, the pH, the rate and extent of concentration, and the presence of other species. The size and nature of these "in-situ" generated sol particles are therefore difficult to control and characterize, and may change with time. The fiber precursors in these processes may thus not be true solutions in that in addition to soluble species they may also contain hydrolyzed or polymerized species of colloidal size.
While it is difficult to generalize because of the number of variables involved, the colloidal particles formed "in-situ" by these processes are amorphous or poorly crystalline species. Regardless of the nature of such colloidal species, the precursor will still contain the bulk of the anions and ligands present from the metal salts prior to concentration. The presence of large quantities of these anions and ligands complicates the firing of the green fibers. In fact, the limitations on these disclosed processes in which the colloidal ZrO.sub.2 species are generated "in-situ" are apparent in the prior art.
Winter et al., (U.S. Pat. No. 3,846,527) disclose a process for the preparation of inorganic fibers by dry spinning a solution, sol or dispersion of one or more metal compounds. The ability to use sols and dispersions of a variety of particles from 5 micrometers to hundreds of angstroms in diameter is discussed. This patent, however, does not disclose fibers of improved properties. Fibers prepared according to the teachings of this patent possess relatively low strengths as disclosed in the examples.
Nowhere does the prior art describe the preparation of strong, flexible, continuous ZrO.sub.2 fibers of greater than about 5 micrometers in diameter. In fact, several unsuccessful attempts to prepare such fibers have been recently described in the literature. Recently published work by Marshall et al., J. Am. Ceram. Soc. 70[8] C-187-C188(1987) on zirconia based fibers prepared from a zirconium acetate based dope indicates that zirconia fibers with high strengths (1.5-2.6 GPa) can be prepared, but only if the fiber diameter is small (less than 5 micrometers). In a subsequent report fibers of up to 12 micrometers in diameter were prepared; however, these fibers broke into smaller fibers during pyrolysis. This study suggested that low density regions produced in the fibers during pyrolysis were the source of the major flaws. See M. E. Khavari, F. F. Lange, P. Smith, and D. B. Marshall, "Continuous Spinning of Zirconia Fibers: Relations Between Processing and Strength" Proceedings of the Materials Research Society; Better Ceramics through Chemistry III, pp. 617-621 (Edited by C. J. Brinker, D. E. Clark, and D. R. Ulrich, Materials Research Society, Pittsburgh, PA (Fall, 1988)). The value that such improved fibers would have in a variety of applications such as reinforcement, filtration, and catalysis is evident to those familiar with the art.